In this invention an electronic regulator has been developed to control the speed of a direct current electric motor, with which is possible to simplify the design regarding the existing velocity regulators.
Equally, it is the purpose of this invention, to reduce the noise produced by the switching, so that the operation of the motor is as silent as possible, and to obtain an acceptable level of electromagnetic interference.
PWM (Pulse Width Modulation) regulators, that are more efficient than lineal regulators, are well known. In the latter the velocity of a direct current motor is controlled by varying the induced current by changing the value of a resistive element (for example a lineal transistor) in series with the motor, and they have the disadvantage of the large losses that take place in this resistive element.
The pulse width modulation regulators (PWM) allow a more effective regulation. These regulators base their operation on a switching device, which might be a power transistor, to which is applied a fixed frequency signal and which is made to always work in one of two states: conduction or non conduction. To vary the current or the output voltage, it modifies the width of the pulse that determines the conduction time of the switching unit. After the switching stage a filter should be placed to eliminate in the amount necessary the current and/or voltage fluctuations caused by the switching.
The use of the PWM regulators has not been introduced definitively in the automobile sector, since it is a solution that implies a high economic cost. The purpose of this invention is to present a product suitable for this and other markets, in which the cost is a key factor.
German patent DE-196.17.947 discloses a power supply circuit for motor minimising mains network reaction. The circuit comprises a rectifier, a controlled switch and a set-point potentiometer. The motor has a free-wheeling diode in parallel with its armature and other stator winding. A further stator winding fulfils the role of a smoothing choke in conjunction with a capacitor.
In this circuit arrangement, the independent use of several stator windings is required for that a modified motor is needed.
The European patent EP-655.835 reveals a method and device for controlling an electric apparatus by means of a pulse width modulation device, in which the electric apparatus is switched on and off at a low cyclic rate within the range of 20 Hz to 10 Khz and wherein each low-frecuency cycle signal (a) is followed by a number of high-frecuency signals (b) in a range of 10 Khz to 100 Khz which delays the current drop.
In this method, interfering low-frecuency noise is prevented due to the sinusoidal current drop af the end of the low-frecuency cycle signal.
The velocity regulator for direct current motors in which this invention consists, presents a simplified design with which the costs are decreased considerably, at the same time that the noises caused by the switching of the motor are reduced, making the switching as smooth as possible.
Thus, the invention consists in that the regulator does not require any inductor element to filter the output of the switching device, since the winding of the motor and its own mechanical inertia function as a filter, with which a significant economic saving is made on eliminating this inductive element.
The key to be able to eliminate the usual inductor in the PWM circuits and to substitute it by the same motor consists in using a switching frequency below audible limit, that is to say below 50 Hz, and to switch smoothly. But since direct current motors act as sound transducers, the switching frequency can be heard if it is in the audible range. In the case of a subsonic modulation frequency, what is heard are the transitions, those alterations that contain audible frequencies. To reduce this effect curving the form of the direct wave has been resorted to, eliminating the usual abrupt transitions in the PWM circuits.
The inductor being eliminated from the PWM circuit, allows this latter to be integrated completely in a semiconductor capsule, something impossible in any other way (the inductor cannot be integrated).
An additional consequence of using smooth switching is that, in many cases, you can do without, as well as the inductor, also the flyback diode usually associated with every PWM circuit. This entails a very important additional cost reduction and the possibility that the circuit may support an inversion of the supply voltage without being destroyed.